Implantable medical sensor and fixation assembly

ABSTRACT

An implantable medical device, such as a sensor for monitoring a selected internally detectable physiological parameter of a patient, is attached to a fixation assembly that is deployable within the patient to position and orient the sensor to enable it to perform its function. The fixation assembly is formed having at least one flexible asymmetric connector where each fixation member includes a plurality of loops, wherein a first loop of the plurality of loops has a maximum pitch that is different from a maximum pitch of a second loop of the plurality of loops. The attachment of the housing and the fixation assembly includes providing a tubular member that is welded to the housing and crimped over a section of the fixation assembly.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/378,989, filed on Dec. 14, 2016 and entitled “IMPLANTABLE MEDICALSENSOR AND FIXATION SYSTEM,” which claims benefit of U.S. ProvisionalApplication Ser. No. 62/267,000, filed on Dec. 14, 2015 and entitled“IMPLANTABLE MEDICAL SENSOR AND FIXATION SYSTEM.” The contents of U.S.application Ser. No. 15/378,989 and U.S. Provisional Application Ser.No. 62/267,000 are incorporated herein by reference in their entirety.

BACKGROUND

Various implantable medical devices have been clinically implanted orproposed for therapeutically treating or monitoring one or morephysiological conditions of a patient. Such devices may be adapted tomonitor or treat conditions relating to heart, muscle, nerve, brain,stomach, endocrine organs or other organs and their related functions.Advances in design and manufacture of miniaturized electronic andsensing devices have enabled development of implantable devices capableof therapeutic as well as diagnostic functions such as pacemakers,cardioverters, defibrillators, biochemical sensors, and pressuresensors, among others. Such devices may be associated with leads toposition one or more electrodes or sensors, or may be leadless, with theability to wirelessly transmit data either to another device implantedin the patient or to another device located externally of the patient,or both.

Although implantation of some devices requires a surgical procedure,other devices may be small enough to be delivered and placed at anintended implant location in a relatively noninvasive manner, such as bya percutaneous delivery catheter. Depending on the nature, function andintended deployment site of the device, the manner in which the deviceis fixed in place and oriented in the body may affect the operation andaccuracy of the device. Consequently, the means by which the device isfixed in place in the body can be a significant factor in itsperformance and utility.

By way of illustrative example, implantable miniature sensors have beenproposed and used in blood vessels to measure directly the diastolic,systolic and mean blood pressures, as well as body temperature andcardiac output. Such direct in vivo measurement of physiologicalparameters may provide significant information to clinicians tofacilitate diagnostic and therapeutic decisions. If linkedelectronically to another implanted therapeutic device (e.g., apacemaker), the data can be used to facilitate control of that device.Such sensors also, or alternatively, may be wirelessly linked to anexternal receiver. As one example, patients with chronic cardiovascularconditions, such as patients suffering from chronic heart failure, maybenefit from the use of implantable sensors adapted to monitor bloodpressures.

SUMMARY

The disclosure describes implantable medical devices, systems, andassociated techniques, structures and assemblies for fixation of theimplantable devices within the body of the patient. In an aspect,fixation assemblies are described that provide both appropriate fixationforce at the implant site and appropriate strain relief for attachmentto the device housing.

The implantable sensor may be coupled to a fixation assembly thatincludes fixation members that are coupled to opposing ends of thehermetic housing, wherein each of the fixation members includes amulti-loop structure with a plurality of loops. A first loop of theplurality of loops has a maximum pitch that is different from a maximumpitch of a second loop of the plurality of loops. In some examples, eachfixation member includes flexible asymmetric loops. The fixationassembly may be formed from a superelastic material and the multi-loopstructure is compressible to a delivery configuration that has anarrower profile in relation to a deployment configuration. Themulti-loop fixation member includes at least two adjacent loops that arecontiguous from a junction in an end-to-end configuration, and at leastone of the loops has a different maximum pitch. In some examples, thepitch of each of the loops decreases towards the junction.

In accordance with some examples, an implantable sensor is attached to afixation assembly of wire-like construction that is compressible todefine a delivery configuration and expandable to a deploymentconfiguration. The delivery configuration defines a pitch, width ordiameter that is narrower, in relation to the deployment configuration,along a common plane. The implantable sensor includes a housing that iscoupled to the fixation assembly in a manner that fixes the position ofthe implantable sensor relative to the axis of the fixation assembly toprevent the sensor housing from rotating about the fixation assembly.

In some examples, the fixation assembly is dimensioned with respect tothe intended deployment site so that when expanded it will contact aportion of the wall of the vessel at substantially diametrically opposedlocations in the vessel to sufficiently maintain the positionalintegrity of sensor at the intended deployment site.

In some examples, the sensor housing may contain pressure sensingcomponents including an externally exposed sensing element and ismounted to the fixation assembly such that, when the fixation assemblyis deployed, the sensing element of the sensor will face along adirection generally perpendicular to the plane of the fixation assembly,so as to be disposed in the vessel lumen and be exposed to the bloodpressure within the vessel.

In a further aspect, a delivery device for the implantable sensor mayinclude a delivery catheter in which the implantable sensor is mountedin its delivery configuration. The implantable sensor is disposed withinthe delivery catheter for delivery of the sensor assembly to the implantsite. When the delivery catheter has been navigated to the intendedimplant site, the implantable sensor is deployed so as to expand to thedeployment configuration to be in contact with the wall of the implantsite and thereby maintain the positional integrity of the sensor at theimplant site.

In one example, an implantable medical device (IMD) includes a housingwith a power source, a sensing element, and an electronic circuit thatis configured to generate a signal indicative of a physiologicalparameter measured by the sensing element. The housing has first andsecond opposing ends. The IMD further includes a fixation assembly withasymmetric fixation members coupled to the opposing ends of the housing.Each of the asymmetric fixation members includes a structure with aplurality of loops. A first loop of the plurality of loops has a maximumpitch that is different from a maximum pitch of a second loop of theplurality of loops.

In another example, an implantable medical system (IMD) includes aphysiological sensor. The physiological sensor includes a housing with apower source, a sensing element, and an electronic circuit. Theelectrical circuit is configured to generate a signal indicative of aphysiological parameter measured by the sensing element. The housing hasfirst and second opposing ends. The IMD includes a fixation assembly.The fixation assembly has asymmetric fixation members coupled to theopposing ends of the housing. Each of the asymmetric fixation membersincludes a structure with a plurality of loops. A first loop of theplurality of loops has a maximum pitch that is different from a maximumpitch of a second loop of the plurality of loops. The IMD includes adelivery catheter having an elongate body for delivery of thephysiological sensor.

In another example, an implantable medical device, includes a housinghaving first and second opposing ends. The IMD includes a pressuresensing element on the housing. The IMD includes an electronic circuitwithin the housing. The electronic circuit may be coupled to thepressure sensing element and configured to generate a signal indicativeblood pressure. The IMD includes a fixation assembly with a firstasymmetric fixation member coupled to the first opposing end of thehousing and a second asymmetric fixation member coupled to the secondopposing end of the housing. Each of the asymmetric fixation membersincludes a structure with a first loop and a second loop. The first loopmay be more proximate to the housing than the second loop. The firstloop has a maximum pitch less than a maximum pitch of the second loop.Each of the fixation members includes first and second free ends withthe first free ends of the fixation members may be oriented in opposingdirections relative to one other. The second free ends of the fixationmembers may be oriented in opposing directions relative to one other.

It should be understood that although the examples described hereinprincipally involve fixing a sensor in a blood vessel, the principlesdescribed herein may be used to make implantable sensors assembliesadapted to measure and monitor any of a variety of physiologicalparameters or to medical devices for delivery of therapy.

BRIEF DESCRIPTION OF THE FIGURES

Throughout the specification, reference is made to the appendeddrawings, where like reference numerals designate like elements.

FIG. 1 illustrates, diagrammatically, a patient with example implantedmedical devices.

FIG. 2A illustrates a side profile view of an example sensor assembly.

FIG. 2B illustrates a side profile view of an example sensor assembly.

FIG. 3A illustrates a bottom perspective view of an example sensorassembly.

FIG. 3B illustrates a side cross-sectional view of an example sensorassembly.

FIG. 4A illustrates an exploded perspective view of an example sensorassembly.

FIG. 4B illustrates a perspective view of an example sensor assembly.

FIG. 5 depicts an example fixation assembly of an example sensorassembly.

FIG. 6A depicts an example fixation assembly of an example sensorassembly.

FIG. 6B depicts an example fixation assembly of an example sensorassembly.

FIG. 7A depicts, diagrammatically and in fragmented illustration, anexample sensor assembly in conjunction with an example delivery system.

FIG. 7B depicts, diagrammatically and in fragmented illustration, anexample sensor assembly in conjunction with an example delivery system.

FIG. 7C depicts, diagrammatically and in fragmented illustration, anexample sensor assembly in conjunction with an example delivery system.

FIG. 8 illustrates, diagrammatically, an example positioning of anexample sensor assembly in a target implant location.

FIG. 9A illustrates a top view of an example fixation assembly for anexample sensor assembly.

FIG. 9B illustrates a top view of an example fixation assembly for anexample sensor assembly.

FIG. 9C illustrates a top view of an example fixation assembly for anexample sensor assembly.

FIG. 9D illustrates a top view of an example fixation assembly for anexample sensor assembly.

FIG. 10 illustrates a side profile view of an example fixation assemblyfor an example sensor assembly.

FIG. 11 illustrates a side profile view of example arrangements of afixation member for an example sensor assembly.

DETAILED DESCRIPTION

The present disclosure describes implantable medical devices (IMDs) thatsense various physiological parameters of a patient, such as bloodpressure. Such IMDs may comprise a fixation assembly and a housing thatcontains a battery and some electronics. The fixation assembly mayinterface with the patient to anchor the device in a stable manner toachieve durable sensing parameters. For proper function, the fixationassembly may be configured for delivery through the vascular structurewhich includes tortuous pathways defined by the blood vessels of thepatient. Therefore, there may be a need for the fixation assembly to fitinto a delivery system, such as a delivery catheter, for delivery, yet,the same fixation assembly needs to provide an appropriate fixation,once deployed in the body, and survive the long-term mechanical loadingat the implant location. In some examples, such as a device having amechanical pressure sensor, an liVID may also be configured to reducethe forces that are transferred to a deformable membrane by thefixation.

This disclosure will describe fixation assemblies in the context of apressure sensing device. However, it should be understood that thefixation assembly may be used in conjunction with other types ofdevices, such as temperature sensors, cardiac output sensors, or therapydelivery devices such as pacemakers and drug delivery devices.

FIG. 1 illustrates, diagrammatically, a patient 2 with implanted medicaldevices including a sensor assembly 10 implanted, for example, in thepatient's pulmonary artery 6 through which blood flows from the heart 4to the lungs, and another medical device 16, such as a pacemaker,defibrillator or the like. For purposes of this description, knowledgeof cardiovascular anatomy is presumed and details are omitted except tothe extent necessary or desirable to explain the context of aspects ofthe disclosure. The device 16 may have one or more leads 18, 20, 22 thatare placed in electrical contact with selected portions of the cardiacanatomy in order to perform the functions of the device 16 as are wellknown to one skilled in the art. The device 16 also may have wirelesscapability to receive and transmit, by telemetry, signals relating tooperation of the device. The device 16 may communicate wirelessly to anexternal device such as a programmer 14 or to another implanted devicesuch as a sensor 12 of the sensor assembly 10. For the sake of clarity,sensor assembly 10 is shown without a fixation assembly in FIG. 1. Thesensor 12 may communicate wirelessly with the programmer 14 or anexternal receiver 24 to provide in vivo data for selected physiologicalparameters to an external site to inform clinicians of the patient'sstatus.

FIGS. 2A, 2B, 3A, 3B, 4A, 4B, 5, 6A, and 6B illustrate examples ofsensor assemblies 10 adapted for minimally invasive placement in apatient's blood vessel. The sensor assemblies 10 are depicted inexamples of deployment configuration (e.g., rather than being depictedin examples of pre-deployment configuration when sensor assemblies 10may be in a sheath). Turning first to FIGS. 2A-2B, side profile views ofthe alternative examples of sensor assembly 10 a and sensor assembly 10b (collectively “sensor assembly 10”) are depicted. The sensor assembly10 includes a sensor 12 coupled to fixation members 30 a, 30 b(collectively “fixation assembly 30”). The fixation assembly 30 andsensor 12 are configured to enable the sensor assembly 10 to be providedin a delivery arrangement that enables the sensor assembly 10 to benavigated to an implant location from which the sensor assembly 10 canbe deployed into the deployment configuration.

As described in this disclosure, it should be understood that thedelivery configuration of the sensor assembly 10 defines a pitch, width,or diameter that is narrower, in relation to the deploymentconfiguration of the sensor assembly 10 along a common plane. As usedherein, delivery configuration may be defined as the general shape ofthe sensor assembly 10 while being delivered to the blood vessel in asheath, specifically as the shape relates to the fixation members 30 a,30 b. Further, the deployment configuration may be defined as thegeneral shape of the sensor assembly 10 while being delivered to theblood vessel in a sheath, once again specifically as the shape relatesto the fixation members 30 a, 30 b. As used herein, pitch refers to theheight that a given loop of the fixation assembly 30 is configured tohave in the deployment configuration, as depicted at least in FIGS. 5and 7A-7C. Put differently, pitch refers to the distance that existsbetween the opposing wire portions within a given loop in a singleplane, wherein the plane is perpendicular to the longitudinal axis 26 ofthe particular sensor assembly 10. Put yet differently, pitch refers tothe length of a line through the central longitudinal axis of a loop,with the line touching two points on opposing edges of each loop, wherethe central longitudinal axis of the loop may be parallel to thelongitudinal axis 26 of the sensor assembly 10.

Upon release/deployment, the fixation assembly 30 expands into thedeployment configuration so as to be in physical contact with the wallof the blood vessel to maintain the positional integrity of sensor 12.In one example, the fixation assembly 30 will engage the interior wallof the vessel defining the blood flow lumen. The sensor 12 may beattached to the fixation assembly 30 in a manner such that the sensingelement 32 of the sensor 12 is spaced from the wall of the vascularlumen to minimize adverse obstruction to blood flow through the lumenand to position the sensing element 32 of the sensor 12 to be fullyexposed to the blood in the vessel, without obstruction from the housingof the sensor or the vessel wall.

In some examples, a bottom longitudinal wall LW2 of the capsule 34 ofthe sensor assembly 10 a may be sintered to promote tissue growth alongthe bottom longitudinal wall LW2. In such examples, the bottomlongitudinal wall LW2 may be sintered as part of a manufacturing step(e.g., the bottom longitudinal wall LW2 may be sintered prior to beingassembled within the capsule 34). In other examples, a bottom portion 48of the sealed housing that includes the bottom longitudinal wall LW2 maybe sintered. Sintering the bottom longitudinal wall LW2 of the capsulemay reduce strain on the fixation members 30 a, 30 b (e.g., as a resultof the sintering providing some fixation force, the fixation members 30a, 30 b may have to provide relatively less fixation forces).

FIG. 3A illustrates a bottom perspective view of the sensor assembly 10a and FIG. 3B illustrates a side cross-sectional view of the sensorassembly 10 a. The features described with respect to sensor assembly 10a and FIGS. 3A and 3B may be included in sensor assembly 10 b. Thesensor 12 includes a capsule 34 that forms a sealed housing thatencloses the operational components such as the electronic circuitry ofthe sensor assembly 10. In some examples, the sealed housing ishermetically sealed. The capsule 34 defines longitudinal walls e.g.,LW1, LW2, that extend from a first lateral side wall SW1 to a secondlateral sidewall SW2. The longitudinal walls define the longitudinalaxis of the sensor 12. As will be described in more detail withreference to FIG. 4, the fixation members 30 a, 30 b may be coupled toan exterior of the capsule 34 such as the first and second sidewalls,respectively.

In some examples, the fixation members 30 a, 30 b may be configured toengage with a vascular wall along a plurality of planes 44 a-d(collectively “planes 44”). The fixation members 30 a, 30 b may expandto occupy the plurality of planes 44 in the deployment configurationafter being released from a sheath as described herein. The fixationmembers 30 a, 30 b may therein have numerous planes of support upondeployment in the blood vessel, which may result in the sensor assembly10 being more resistant to “twisting” in a direction generallyperpendicular to one of the planes 44 (e.g., more resistant incomparison to an example sensor assembly 10 with a respective fixationassembly 10 that exists along a single plane).

Fixation assembly 30 may apply little more than the force that isappropriate to hold the sensor assembly 10 in place without applyingexcessive force to that surface. The fixation assembly 30 may beconstructed to apply light, but sufficient, force to the vessel. Suchforces are at least less than those associated with the placement ofvascular stents in which the objective may be to press against thevascular wall with sufficient force to provide scaffolding support forthe vessel wall.

FIGS. 4A and 4B are exploded perspective views of the sensor 12 inaccordance with some examples. The capsule 34 may include an elongatebody that defines an interior cavity. The interior cavity of the capsule34 may be sized and shaped to contain the battery 40 and electronics andsensor components 42 of the sensor 12. The capsule 34 may be designedwith shapes that are easily accepted by the patient's body whileminimizing patient discomfort. For example, the body of capsule 34 maybe formed in a cylindrical shape with cylindrical sidewalls. Othernon-cylindrical configurations may be employed, however, in which casethe corners and edges may be designed with relatively large radii topresent a capsule having smoothly contoured surfaces. In the depictedexample, the body of capsule 34 may be formed as a generally rectangularstructure, which means that the outline of the shape of capsule 34resembles a rectangle as defined by two lateral side walls SW1, SW2 andtwo longitudinal side walls LS1, LS2, with contoured edges and corners.

The capsule 34 may be formed having two sections 36, 38. In someexamples, one section contains and/or supports the sensing element 32while the other section contains and/or supports components operablyconnected to the sensing element. For example, section 36 may containand/or support a pressure sensing diaphragm of sensor 12 and sensorcomponents 42 while section 38 contains the battery 40.

In some examples, the fixation members 30 a, 30 b have opposingarrangements as reflected over a central plane 54 of the sensor 12. Thefixation members 30 a, 30 b having opposing arrangements as reflectedover the central plane 54 may include specific features of the fixationmembers 30 a, 30 b (e.g., a relative rise or dip of a wire along thelongitudinal axis 26 of a respective fixation member) beingsubstantially opposite on a relative side of the sensor 12. For example,fixation assemblies 30 may include near portions of wire 56 a-b(collectively “near portions of wire 56”) that extend axially out fromthe capsule 34 closer to a first longitudinal side wall LS1 (e.g.,closer to the depicted vantage point) than far portions of wire 58 a-b(collectively “far portions of wire 58”). Put differently, as usedherein, a near portion of wire 56 of a respective fixation member 30indicates the portion of wire of said fixation member 30 that is infront of a respective far portion of wire 58 of the fixation member 30as depicted at a juncture 52 (e.g., where a near portion of wire 56obscures the far portion of wire 58 at the juncture 52). As depicted inFIG. 4A, near portion of wire 56 a initially dips as it projects axiallyout from the capsule 34 before rising to a junction 52, while nearportion 56 b initially rises as it projects axially out from the capsule34 before dipping to a respective juncture. Similarly, far portions 58of the fixation elements 30 a, 30 b have opposite arrangements asreflected over a central plane 54. In certain examples, fixation members30 a, 30 b having opposing arrangements as reflected over a centralplane of the sensor 12 may result in load balancing benefits (e.g., asthe two fixation members 30 a, 30 b are configured to stabilize indifferent directions against different rotations), which may result in asensor assembly 10 being more stably deposited into a blood vessel.

The capsule 34 may be formed from one or more biocompatible materialsthat can be sealed (e.g., hermetically sealed) when the sections 36, 38are joined. A number of such biocompatible materials may be employed, aswill be understood by skilled in with the art, including metals andbiocompatible plastics. For example, the sections 36, 38 may be formedfrom unalloyed titanium with an American Society for Testing andMaterials (ASTM) grade between 1 and grade 4, or the sections may beformed from an alloyed titanium (e.g., grade 5) that includes aluminumand vanadium. In other examples, section 36 may be formed from abiocompatible mineral, such as sapphire or another variety of corundum.For some examples in which the sections are metal, the metal material ofthe capsule 34 may optionally be selected to be compatible with thefixation assembly 30 material so as to permit the fixation assembly 30being securely coupled to the capsule 34. In other examples, the capsule34 along with the fixation assembly 30 may be integrally formed from oneor more of the same or distinct materials. In some examples, the capsule34, as well as some portions of the fixation member 30, may beencapsulated in a biologically inert dielectric barrier material such asa film of silicone or polyp-xylylene) polymer sold under the trademarkPARYLENE.

As shown in FIG. 4A, capsule 34 may include fasteners F1, F2 that definechannels for reception of a segment of the fixation assembly 30. In theexample of FIG. 4B, capsule 34 may include fasteners F3, F4 that definechannels for reception of a segment of the fixation assembly 30. Thereceived segment may include a portion along a length of the fixationassembly 30 or a free end of the fixation assembly 30. The fastenersF1-F4 may be coupled to an exterior of the capsule 34, or in alternativeexamples, formed integrally with the capsule 34. For example, as shownin FIG. 4A, the fasteners F1, F2 are provided at an exterior of thecapsule 34 at the lateral sidewalls SW1, SW2, respectively. In thealternative example within FIG. 4B, the fasteners F3, F4 are provided atspaced apart locations on an exterior of one or more of the longitudinalwalls of the capsule 34, such as on the top longitudinal wall LW1, asdepicted in FIG. 4B, or alternatively on the bottom longitudinal wallLW2. Spaced apart locations, as used herein, may include four locationson a longitudinal wall LW1, LW2 of the capsule 34, where a first twospaced apart locations are a first distance away from each other on afirst lateral sidewall SW1, and the second two spaced locations aremirrored across the central plane 54 of the sensor assembly 10 the firstdistance away from each other on the second lateral sidewall SW2.

In some examples, the fasteners F1-F4 are formed as pairs of tabs thatare arranged to define one or more channel(s) for receiving one or moresegment(s) of the fixation assembly 30. Each fastener F1-F4 can includea pair of tabs that are aligned longitudinally as described, forexample, in U.S. Pat. No. 8,864,676 to Beasley et al., which isincorporated herein by reference in its entirety. The fasteners F1-F4may be coupled to the capsule 34 through welding, for example.Alternatively, the fasteners F1-F4 may be formed integrally with thecapsule 34. In some examples, the fasteners F1-F4 may be on opposingends of the capsule 34. It is to be understood that the description ofthe fasteners F1-F4 is not intended to be limiting, and rather, it isprovided to explain the context of aspects of the disclosure.

In the examples depicted in FIGS. 4A-4B, the fasteners F1-F4 are formedas tubular structures that define channels that are sized to receive asegment of each of the fixation members 30 a, 30 b. In accordance withsome examples, the fasteners F1-F4 may be formed as discrete components,such as tubes, for example, that can be coupled to the capsule 34through coupling techniques such as welding or bonding agent such asglue or crimping. Alternatively, the fasteners F1-F4 may be formedintegrally with the capsule 34. As will be described in more detailbelow, the fixation assembly 30 may be coupled to the fasteners F1-F4 byany suitable coupling technique such as welding, crimping, bonding agentsuch as glue, or frictional fit.

The channels of fasteners F1-F4 may optionally be defined to receive asegment of the fixation members 30 a, 30 b in a snug fit arrangement toprevent relative movement between the capsule 34 and the fixationassembly 30. By way of dimensional example, the thickness of a crosssection of fixation assembly 30 may be on the order of 0.006 inch for around shape or 0.0053 inch by 0.012 inch for a rectangular shape. Incomparison, the diameter (or width) of the channel of each of thefasteners may be on the order of 0.010 inch to 0.025 inch.

As used herein, free ends 68 a-d (collectively, “free ends 68”) of afixation member 30 a, 30 b may be the two terminating points of the wireof a respective fixation member 30, 30 b which may therein each beconnected to the capsule 34. The free ends 68 of each of the fixationmembers 30 a, 30 b may be oriented in opposing directions. For example,a first free end 68 a, 68 c may be oriented downward in relation to thelateral sidewall SW1, SW2, while the other ends 68 b, 68 d may beoriented upward in relation to the lateral sidewalls SW1, SW2 as shownin FIG. 4A. Among other things, such an orientation can provide a degreeof load cancellation that minimizes load transfer to the sensing element32. In alternative examples, one of the fixation members e.g., 30 a maybe coupled along a lateral sidewall such as SW1 as shown in FIG. 4A, andthe other of the fixation members e.g., 30 b may be coupled to alongitudinal wall such as LW1 or LW2 as shown in FIG. 4B.

FIG. 5 depicts the fixation assembly 30 of the sensor assembly 10. Forease of discussion, the details of sensor 12 are not shown and anoutline of the sensor 12 is shown in dashed line. The reader may referto the foregoing figures in conjunction with the description of FIG. 5.Each of the fixation members 30 a, 30 b comprises a flexible materialand may be configured in a helical configuration in one examples. Eachof the fixation members 30 a, 30 b may be coupled at opposing ends ofthe sensor capsule 34. In the illustrated example, the fixation members30 a, 30 b are coupled at opposing lateral sidewalls of capsule 34(e.g., as depicted with respect to 10 a and 2 a) The configuration of 30a, 30 b, depicted and described with respect to FIG. 5 may be equallyapplicable to other example locations of the attachment of the fixationstructures to capsule 34, such as attachment to of fixation members 30a, 30 b to top longitudinal wall LW1 as depicted in FIG. 2b . In someexamples, the fixation assembly 30 may be formed from a highly elasticmaterial capable of “remembering” a first shape, such that even when thefixation assembly 30 is condensed to a second, smaller shape (e.g., whenthe fixation assembly 30 is inside a sheath) the fixation assembly 30may return to the first shape when no longer so condensed (e.g., whenremoved from the sheath). For example, the fixation assembly 30 may beformed from a highly elastic biocompatible alloy capable of formingstress induced martensite (SIM). Nitinol (TiNi) is an example of suchmaterials that are also referred to as being “pseudoelastic” or“superelastic.” Each of the fixation members 30 a, 30 b can be formedinto a single, integral component.

In some examples, the fixation members 30 a, 30 b may be formed from awire-like element that is configured into the desired shape. Suchwire-like elements may comprise a linear element having any desiredcross-section such as round or rectangular. In other examples, thefixation members 30 a, 30 b, may be formed from a sheet of material bylaser cutting or electrochemical etching or other fabricating techniquesknown in the art. Regardless of the construction method, each of theresulting fixation members 30 a, 30 b may have a substantially uniformthickness. As used in this disclosure, the term substantially uniformthickness means that the thickness dimension along a length of themembers 30 a, 30 b is constant or is within a variation of up to 15%.

Each one of the fixation members 30 a, 30 b may be configured to definea pair of longitudinally spaced asymmetric loops 50 a, 50 b formed in ahelical configuration when attached to capsule 34. The asymmetric loops50 a, 50 b are formed in an end-to-end configuration so as to intersector overlap at junctions 52. It should be understood that the asymmetricloops 50 a, 50 b need not be in contact at the junctions 52 (e.g., as aresult of being in different planes 44 to have numerous planes 44 ofsupport as discussed herein), but rather, that they may overlap asviewed from the side to form a helical configuration as shown in theperspective view of FIG. 3. Stated another way, the cross section of thestructure of each of the fixation members 30 a, 30 b may generallyresemble the number “8” in some examples when viewed from a sideprofile. Moreover, although two loops are depicted, the fixation members30, 30 b may include multi-loop structures including three or more loopsas depicted, for example, in FIGS. 6A, 6B.

A length of the fixation members 30 a, 30 b may be contiguous.Alternatively, a length of the fixation members may be detached around aperimeter of one of the loops away from the junction. It should beunderstood that either one or both fixation members 30 a, 30 b may becontiguous or detached around the perimeter. A fixation member 30 a, 30b that is detached may include two discrete portions of wire that areeach individually coupled to the capsule 34 in a manner consistent withthis disclosure. A fixation member 30 a, 30 b that is detached maymaintain substantially similar shapes as other fixation members 30 a, 30b discussed herein. Fixation members 30 a, 30 b that are detached mayhave “breaks” 35 a-35 b (collectively “breaks 35”) at a locationfurthest away from the capsule 34 along the longitudinal axis 26 asdepicted in FIG. 3A. For example, in a fixation member 30 a, 30 b thatis detached, a far portion of wire 56 a may not be part of the same wireas near portion of wire 56 b, but instead the far portion of wire 56 aand near portion of wire 56 b may be two discrete portions of wire thatterminate at substantially the same spot (e.g., at the break 35) along afixation member 30 a, 30 b. In some examples, the two discrete portionsof wire may terminate at the break 35 such that a fixation member 30 a,30 b that is detached may appear to be a single piece of wire (e.g., itmay appear to be contiguous) before a relatively large force is appliedupon the respective fixation member 30 a, 30 b and causes the discreteportions of a fixation member 30 a, 30 b that is detached to separate atthe break 35. In some examples, a fixation member 30 a, 30 b that isdetached may encounter relatively less strain upon the discrete wireportions as a result of being detached.

As is depicted in FIG. 5, the asymmetric loop 50 a may be formed havinga maximum pitch P1 that is of a lesser magnitude than a maximum pitch P2of the asymmetric loop 50 b. As used herein, a maximum pitch of a loopis the pitch of the largest magnitude of the respective loop. The pitchof loop 50 a decreases in a direction towards the junction 52 along alongitudinal axis 26 of the sensor assembly 10 a (or increases in adirection away from the junction 52). Similarly, the pitch of loop 50 bdecreases in a direction towards the junction 52 along a longitudinalaxis 26 of the sensor assembly 10 (or increases in a direction away fromthe junction 52). In examples having more than two loops, each of theadditional loops may likewise be formed with decreasing pitches,relative to a junction 52 of such additional loop(s) to one of theadjoining loop(s).

In some examples, the maximum pitches P1, P2 may be configured toimprove a fit of the fixation members 30 a, 30 b in the blood vessel.For example, P2, being larger than P1, may be configured to be slightlygreater (e.g., 10% more) than the height of the respective blood vesselthat the sensor 10 assembly may be implanted in, such that the fixationmembers 30 a, 30 b engage with but do not pierce/push through the wallsof the blood vessel. Further, the radius of the wires of the fixationmembers 30 a, 30 b may be as large as possible (e.g., while maintainingshape memory and proper alignment) to minimize loading on the capsule 34and therein the sensor 12.

The fixation members 30 a, 30 b are each coupled to the capsule 34 at asegment of the first loop 50 a. As such, the fixation members 30 a, 30 bmay be coupled to the capsule 34 such that the pitch of the first loop50 a decreases along a longitudinal axis 26 of the housing towards thejunction 52. In some examples, the pitch of the first loop 50 aincreases for a relatively short distance 46 a-b (collectively“distances 46”) along the longitudinal axis before the pitch decreases.Conversely, the pitch of the second loop increases along thelongitudinal axis 26 away from the junction 52. Moreover, the fixationmembers 30 a, 30 b may be compressible along a dimension defining thepitch of the first loop 50 a and second loop 50 b such that each memberis collapsible to a reduced pitch in a delivery configuration andexpanded to an magnified pitch in a deployment configuration.

In some examples, some or all of the wire of the fixation assembly maybe coated (e.g., coated for insulation purposes) or otherwise coveredwith paralyne or another insulating material. In other examples, wiresof the fixation members 30 a, 30 b may be exposed (e.g., without anelectrical insulator around all or part of the conductor of a wire) tocreate electrical contact with tissue of the patient 2. The fixationmembers 30 a, 30 b may be configured to make electrical contact withtissue of the patient 2 in order to transmit signals through the tissueof a patient 2 (e.g., signals to a medical device 16 or a programmer 14or an external receiver 24). In some examples, wires may be strippedafter the maximum pitch P1 of the first loops 50 a. Put differently, insome examples, all of the wire of the fixation members 30 a, 30 b may bestripped with the exception of the wire within the distances 46 of thecapsule 34. In such cases, stripping the wires after the maximum pitchP1 of the first loops 50 a may provide transmission strength benefits tothe sensor assembly 10.

The fixation assembly 30 may stably position the sensor 12 to achievestable and durable sensing parameters. Further, the fixation assembly 30may reduce the loads that are transferred by the fixation assembly 30 tothe sensing element 32. In some examples, the sensing element 32 is adeformable pressure membrane. Reducing the loads that are transferred tothe sensing element may be achieved by providing a fixation assembly 30having a multi-loop configuration such that at least one of the loopsprovides strain relief for coupling to the capsule 34 while a second ofthe loops provides the fixation to maintain the positional integrity ofthe sensor 12 at the desired implant location. In some examples, thefixation assembly 30 can be constructed such that an outer perimeter ofeach of the fixation members 30 a, 30 b is aligned with a plane definedby an exterior portion of the capsule 34. Such a plane can be defined bythe bottom longitudinal section of capsule 34 as shown in FIG. 5. Insome examples, an arrangement enables the bottom of the capsule 34 to bein contact or adjacent to the wall of the vessel during use whilepositioning the sensor in the blood flow path within the vessel. Such aconstruction also provides for unobstructed passage of a guidewirewithin the lumen of the delivery tool during the implantation of thesensor assembly 10.

In the illustrative example, the fixation members 30 a, 30 b are coupledat two separate locations on opposing ends of the capsule 34. Thisprovides transverse stability of the capsule 34. In some examples,coupling the fixation members 30 a, 30 b to the two opposing ends ofcapsule 34 may provide a fixation structure with a decreasing pitch inopposing directions, which further minimizes the load transfer to thesensing element 32. Furthermore, although the direction of the couplingof fixation assembly 30 is depicted as being parallel with thelongitudinal axis of the capsule 34, it should be understood thatalternative examples may selectively couple the fixation assembly 30 ina different orientation relative to the capsule 34. For example, thefixation assembly 30 may be coupled perpendicular to the longitudinalaxis 26 of the capsule 34.

FIGS. 7A-7C depict, diagrammatically and in fragmented illustration, thesensor assembly in conjunction with an example delivery system that maybe used to deliver and deploy the sensor assembly in a desired implantlocation. The fixation members 30 a, 30 b are formed from a flexiblematerial that enables the fixation assembly 30 to be compressed to anarrower shape having a smaller effective cross section in which it maybe mounted to and delivered by a delivery catheter 60 to an intendedimplant location such as the pulmonary artery.

In one configuration, the larger of the loops 50 of each of the fixationmembers 30 a, 30 b is compressed from its relaxed, expanded deploymentconfiguration to a narrower, more elongated delivery configurationdefined by loop segments that are drawn more closely parallel to eachother. In this respect, forming fixation members 30 a, 30 b from asuperelastic material (e.g., such as superelastic nitinol, which has theability to undergo extreme strain without permanent deformation) reducesthe risk of permanent deformation when the loops are compressed.Nevertheless, other materials such as stainless steel or plastic maysuitably be used to form the fixation members 30 a, 30 b. In yet anotherconfiguration, each of loops 50 of the fixation members 30 a, 30 b maybe compressed from a relaxed, expanded shape to a narrower, moreelongated shape.

Turning to FIGS. 7A-7B, the fixation assembly 30 is shown with thefixation members 30 a, 30 b in a compressed, delivery configurationprofile as may be the case while disposed within the delivery catheter60. The delivery configuration enables the sensor assembly 10 to bedelivered to a desired implant location through a delivery catheter 60.FIG. 7A shows the fixation assembly 30 in a delivery configuration suchthat at least the dimension along the maximum pitch P2 of the secondloop 50 b is reduced to define a low profile of fixation members 30 a,30 b. Depending on the size of the delivery catheter 60, the fixationassembly 30 may be compressed such that the dimensions of both themaximum pitch P1 of the first loop 50 a and the maximum pitch P2 of thesecond loop 50 b are reduced to define a low profile of fixation members30 a, 30 b.

In FIG. 7B, the fixation assembly 30 is shown partially retracted fromthe delivery catheter 60 such that the fixation member 30 b is in theexpanded, deployment configuration such that the dimension along themaximum pitch P1 of the first loop 50 a and/or the maximum pitch P2 ofthe second loop 50 b is expanded to its deployed configuration.

The delivery catheter 60 may be in the form of an elongate tubular shaft62 having proximal end 64 and distal end 66 with the sensor assembly 10disposed within a region of the shaft. The shaft 62 may be formed from amaterial and dimensioned to have sufficient flexibility to be navigatedthrough the patient's vasculature to the intended implant location. Thedelivery catheter 60 may further include a guide sheath or used inassociation with a guide wire, as is known to one skilled in the art.The sensor assembly 10 may be releasably retained at the region of theshaft by any suitable arrangement, such as the rotatable helicalretention elements described in U.S. Pat. No. 8,864,676.

The delivery catheter 60 may be advanced through a guide sheath that,when retracted, exposes the sensor assembly 10 at a desired implantlocation. In alternative examples, the delivery catheter 60 may beadvanced through an introducer to the desired implant location. Once thedistal end 66 is positioned near the implant location, the sensorassembly 10 may be deployed by advancing the distal end 66 to deploy thesensor assembly 10. As the sensor assembly 10 is released itself-expands to its expanded configuration within the target implantlocation.

In FIG. 7C, the entire sensor assembly 10 is shown in an expandeddeployment configuration profile, which typically occurs followingrelease of the fixation members 30 a, 30 b from the delivery catheter60. The sensor assembly 10 is typically advanced from the deliverycatheter 60 and the expansion of the loop 50 b causes the sensorassembly 10 to be securely positioned at the target implant location.Repositioning may be accomplished by advancing the delivery catheter 60to recapture the fixation members 30 a, 30 b. The recaptured sensorassembly 10 may then be repositioned and redeployed.

The delivery catheter 60 may be advanced to the target implant locationby advancing it through a guide sheath, an introducer, a guide wire inan over-the-wire system, or any other mechanism which is known to thoseskilled in the art. It should be understood that delivery catheter 60 isonly one example of a delivery system for sensor assembly 10. Othertypes of delivery systems can be utilized, including, for example,mechanisms that are slidably disposed around the sensor assembly 10 toconstrain the sensor assembly in its delivery configuration until apusher mechanism ejects the sensor assembly 10 from the distal end ofthe catheter. It should be noted that the superelastic construction ofthe fixation members 30 a, 30 b enables the fixation members 30 a, 30 bto be elastically distorted from respective planar expanded shape to ashape adapted to fit onto or within a delivery catheter.

FIG. 8 illustrates, diagrammatically, the positioning of the sensorassembly 10 in a target implant location. In the depicted example, theimplant location is a human pulmonary artery 6, which is generallyrelatively short and often has a lumen that tapers in the direction ofblood flow. The degree of taper may vary from patient to patient, withpatients suffering from chronic heart failure tending to have moresevere taper with higher pulmonary blood pressures. The main pulmonaryartery branches into left and right pulmonary arteries 8, 9. Whether theclinician will elect to place a device in the main artery or one of thebranches of the pulmonary artery tree will depend on the anatomy andcondition of the particular patient among other factors.

When deploying the sensor assembly 10, the delivery catheter 60 may bepositioned so that the more distal of fixation members 30 a, 30 b willbe located in the selected portion of the selected artery. Fixationassembly 30 may apply little more than the force that is required tohold the sensor assembly 10 in place without applying excessive force tothat surface. The fixation assembly 30 is constructed to apply light,but sufficient, force to the vessel. Such forces are at least less thanthose associated with the placement of vascular stents in which theobjective is to press against the vascular wall with sufficient force toprovide scaffolding support for the vessel wall. By contrast, aspects ofthe disclosure intend to maintain the sensor assembly 10 in the vessel,without migrating upstream or downstream, while supporting the sensor 12in its intended position and orientation for measurement of stable anddurable sensing parameters. When the sensor assembly 10 is deployed, thefixation members 30 a, 30 b expand along a single plane with at leastone loop of each fixation member 30 a and 30 b expanding to a dimensionto be in contact with the luminal wall of the vessel. Regardless of theorientation of the sensor assembly 10 during delivery, the at least oneloop that is in contact with the vessel wall will seat itself atsubstantially diametrically opposite surfaces of the vessel wall (e.g.,at least one loop of each fixation member 30 a, 30 b may press into bothsides of a vessel wall). As used in this disclosure, the termsubstantially diametrically opposite may mean that the surfaces areopposite one another or within a 15% variance of being opposite eachother. Among other things, such a construction can enable the at leastone loop to maintain the positional integrity of the sensor assembly 10with respect to the vessel. In that deployed position, the sensingelement 32 may be oriented along a longitudinal axis in relation to thelength of the vessel lumen to be exposed fully and without obstructionto blood flow in the lumen.

In other examples, the fixation assembly and sensor are arranged suchthat the sensing element faces generally parallel to the plane of thefixation assembly. The fixation assembly also may be configured toposition the sensor housing and, particularly, the sensing element, awayfrom the vessel wall to lessen the risk of turbulent flow through thevessel.

FIGS. 9A-D illustrate top views of example fixation members 70, 76, 82,88 for an example sensor assembly (not depicted). FIGS. 9A-D depictsfixation members 70, 76, 82, 88 from the same view as FIG. 3A. Fixationmembers 70, 76, 82, 88 includes an angled portion 74, 80, 86, 92(respectively) and free ends for connecting the respective fixationmembers 70, 76, 82, 88 to respective capsules 34. The specific angles ofthe fixation members 70, 76, 82, 88 are depicted for purposes ofillustration only, other angles and configurations are also possible.For example, in some instances, fixation members 70, 76, 82, 88 couldhave a more acute angle similar to the angle depicted in FIG. 3A. Thefixation members 70, 76, 82, 88 may have two loops, three loops, or morethan three loops.

Fixation member 70 includes an angled portion 74 and free ends 72 a-b(collectively “free ends 72”). The free ends 72 may be parallel with anaxis 26 of the sensor assembly 10. The amount of the fixation assembly70 that is comprised of the free ends 72 is for example purposes only;in other examples, the free ends 72 may comprise a smaller or largeroverall amount of the fixation member 70. The free ends 72 may connectto the capsule 34 as described herein. In some examples, it may beeasier to attach the fixation member 70 to the capsule 34 due to thefree ends 72 lining up with relative components (e.g., fasteners) of thecapsule 34.

Fixation member 76 includes an angled portion 80 and two free ends 78a-b (collectively “free ends 78”). The free ends 78 may be parallel withplanes 84 a-b (collectively “planes 84”) of the fixation member 76.Planes 84 of the fixation member 76 may be substantially similar toplanes 44 of the fixation members 30 a, 30 b as described herein. Thefree ends 78 may connect to a capsule 34 as described herein. In someexamples, fixation member 76 may experience benefits in balancing loadsthroughout the fixation assembly 76 being as there are no turns intransitioning to the free ends 78 (as seen from the top) in which loadsmay be concentrated. Further, in certain examples, it may be easier/lessexpensive to manufacture fixation member 76 than other fixation members,as less turns are required.

Fixation member 82 includes an angled portion 88 and free ends 86 a-b(collectively “free ends 86”). The length of the free ends 86 is forexample purposes only; in other examples, the free ends 86 may be longeror shorter. The free ends 86 may connect to the capsule 34 as describedherein. The free ends 86 may parallel to a respective lateral sidewallSW1, SW2 of the capsule 34. In some examples, it may be easier to attachthe fixation member 82 to the capsule 34 due to the free ends 86 liningup correctly with relative components (e.g., fasteners) of the capsule34. Further, in some examples, loads upon the angled portion 88 may berelatively lower, as loads do not transfer efficiently from the freeends 86 to the angled portion 88 due to the free ends being parallelwith a respective lateral wall SW1, SW2.

Fixation member 90 includes an angled portion 94 and free ends 92 a-b(collectively “free ends 92”). The length and angle of the free ends 92is for example purposes only; in other examples, the free ends 92 may belonger or shorter at different angles. The free ends 92 may connect tothe capsule 34 as described herein. The free ends 92 may curve into thecapsule with a radius 96. In some examples, loads upon the angledportion 94 may be relatively lower, as loads do not transfer efficientlyfrom the free ends 92 to the angled portion 94 due to the curve with theradius 96.

FIG. 10 illustrates a side profile view of an example fixation member100 for an example sensor assembly (not depicted). All dimensions are inmillimeters and are for purposes of example only; other dimensionsconsistent with this disclosure are also possible. A connection portion102 of the fixation member 100 may incorporate any of the connectionconfigurations discussed in FIGS. 9A-9D.

FIG. 11 illustrates a side profile view of example configurations of afixation member 110 for an example sensor assembly (not fully depicted).Fixation member 110 may attach to a capsule 34 as depicted. In otherexamples, fixation member attaches to a capsule 34 in other mannersconsistent with this disclosure (e.g., as depicted in FIG. 4A). Fixationmember 110 has a first loop 124 and a second loop 126. In otherexamples, fixation member 110 has more than two loops as describedabove.

A first loop 124 of the fixation member 110 may be configured with anyone of a plurality of maximum pitches 120 a-d (collectively “maximumpitches 120”). The first loop 124 may be configured with to have one ofa plurality of maximum pitches 120 as a result of altering arrangementsof far portions of wire 58 that is closer to longitudinal side wall LS2than longitudinal side wall LS1. The far portion of wire 58 may bearranged differently immediately upon extending axially out from thecapsule 34. For example, the far portion of wire 58 may be arranged inthe arrangements 112, 114, 116, 118 depicted in FIG. 11. In otherexamples, the fixation member 110 may be arranged in other manners thatare consistent with this disclosure (e.g., the far portion of wire 58may arranged to occupy space between arrangement 114 and arrangement118).

In some examples, an arrangement 112 of the far portion of wire 58 mayinclude the far portion of wire 58 rising up (e.g., moving axially outfrom the capsule 34 in the general direction of the longitudinal wallLW1 relative to the capsule 34) such that the pitch of the first loop124 increases relatively quickly to a maximum pitch 120 a and thendecreases to the juncture 52. In other examples, an arrangement 116 ofthe far portion of wire 58 may include the far portion of wire 58 risingup relatively moderately such that the pitch of the first loop 124increases to a to a maximum pitch 120 b (e.g., where maximum pitch 120 bis less than maximum pitch 120 a) and then decreases to the juncture 52.In other examples, an arrangement 118 of the far portion of wire 58 mayinclude the far portion of wire 58 dipping down (e.g., moving axially inthe general direction of the longitudinal wall LW2 relative to thecapsule 34) before rising slightly to the maximum pitch 120 c (e.g.,where maximum pitch 120 c is less than maximum pitch 120 b), such thatthe pitch of the first loop 124 slightly increases until the maximumpitch 120 c and then slightly decreases until the juncture. In otherexamples, an arrangement 114 of the far portion of wire 58 may dip downat a slower rate than the near portion of wire 56 dips down, such thatthe first loop 124 has a maximum pitch of 120 d (e.g., where the maximumpitch 120 d is less than the maximum pitch 120 c).

In some examples, a larger maximum pitch 120 of the first loop 124 mayprovide more stiffness to the fixation assembly 124. For example,arrangement 112 may be relatively more stiff than arrangement 116 (e.g.,as maximum pitch 120 a is larger than maximum pitch 120 b), whilearrangement 116 is relatively more stiff than arrangement 118, whilearrangement 114 is relatively more stiff than arrangement 118. In suchexamples, it may be advantageous for a fixation member 110 to be stiffenough to maintain a shape and engage walls of a blood vessel while notbeing so stiff as to immediately or eventually push through walls of ablood vessel. As such, different arrangements 112, 114, 116, 118 may beutilized for different applications depending upon the stiffnessrequired for the specific parameters of the respective application.

The following paragraphs include examples (enumerated consecutively from1 to 34) that provide for various aspects of the present disclosure. Inone example of a first paragraph (1), an implantable medical devicecomprises:

a housing including a power source, a sensing element, and an electroniccircuit configured to generate a signal indicative of a physiologicalparameter measured by the sensing element, the housing having first andsecond opposing ends; and

a fixation assembly including asymmetric fixation members coupled to theopposing ends of the housing, wherein each of the asymmetric fixationmembers includes a structure with a plurality of loops, wherein a firstloop of the plurality of loops has a maximum pitch that is differentfrom a maximum pitch of a second loop of the plurality of loops.

2. The implantable medical device of paragraph 1, wherein each loop ofthe structure is formed in a helical configuration.

3. The implantable medical device of any of paragraphs 1-2, wherein thefirst loop is coupled to the housing such that a pitch of the first loopincreases from the junction towards the housing and a pitch of thesecond loop increases in a direction away from the junction.

4. The implantable medical device of any of paragraphs 1-3, wherein eachof the asymmetric fixation members is configured in a figure-of-eightstructure with each of the loops of the figure-of-eight structure havinga different maximum pitch.

5. The implantable medical device of any of paragraphs 1-4, wherein eachof the asymmetric fixation members is configured to contact the walls ofa blood vessel along a plurality of planes.

6. The implantable medical device of any of paragraphs 1-5, wherein eachplane of the plurality of planes is perpendicular to a surface of thecapsule to which the fixation members are affixed.

7. The implantable medical device of any of paragraphs 1-6, wherein eachof the fixation members includes first and second free ends, wherein thefirst and second free ends are parallel with a plane of the plurality ofplanes.

8. The implantable medical device of any of paragraphs 1-7, wherein theasymmetric fixation members have opposite arrangements as reflectedacross a central plane of the implantable medical device.

9. The implantable medical device of any of paragraphs 1-8, wherein atleast one loop of the structure is dimensioned having a diameter tocontact a portion of a wall of a vessel to thereby maintain the pressuresensor at a fixed location within the vessel.

10. The implantable medical device of any of paragraphs 1-9, wherein thevessel is a pulmonary artery.

11. The implantable medical device of any of paragraphs 1-10, wherein apitch of each loop increases from the junction along an axis that isparallel to a longitudinal axis of the housing.

12. The implantable medical device of any of paragraphs 1-11, whereinthe sensing element is a pressure membrane and the measuredphysiological parameter is blood pressure.

13. The implantable medical device of any of paragraphs 1-12, whereinthe housing further comprises an electronic circuit configured togenerate a signal indicative of the physiological parameter measured bythe sensing element.

14. The implantable medical device of any of paragraphs 1-13, whereinthe structure has a variable pitch such that the fixation assembly iscompressible in a delivery configuration and expandable into adeployment configuration that is different from the deliveryconfiguration.

15. The implantable medical device of any of paragraphs 1-14 whereineach of the fixation members includes first and second free ends withthe first free ends of the fixation members being oriented in opposingdirections relative to one other and the second free ends of thefixation members being oriented in opposing directions relative to oneother.

16. The implantable medical device of any of paragraphs 1-14, whereineach of the fixation members includes first and second free ends,wherein the first and second free ends are parallel with a longitudinalaxis of the implantable medical device.

17. The implantable medical device of any of paragraphs 1-14, whereineach of the fixation members includes first and second free ends,wherein the first and second free ends are perpendicular with alongitudinal axis of the implantable medical device.

18. The implantable medical device of any of paragraphs 1-17, wherein asurface of the housing that contacts a wall of a blood vessel issintered.

19. An implantable medical system, comprising:

a physiological sensor including:

a housing including a power source, a sensing element, and an electroniccircuit configured to generate a signal indicative of a physiologicalparameter measured by the sensing element, the housing having first andsecond opposing ends; and

a fixation assembly including asymmetric fixation members coupled to theopposing ends of the housing, wherein each of the asymmetric fixationmembers includes a structure with a plurality of loops, wherein a firstloop of the plurality of loops has a maximum pitch that is differentfrom a maximum pitch of a second loop of the plurality of loops; and

a delivery catheter having an elongate body for delivery of thephysiological sensor.

20. The implantable medical device of paragraph 19, wherein the sensingelement is a pressure membrane and the measured physiological parameteris blood pressure.

21. The implantable medical device of any of paragraphs 19-20, whereineach loop of the structure is formed in a helical configuration.

22. The implantable medical device of any of paragraphs 19-21, whereineach of the asymmetric fixation members is configured in afigure-of-eight structure with each of the loops of the figure-of-eightstructure having a different maximum pitch.

23. The implantable medical device of any of paragraphs 19-22, whereinat least one loop of the structure is dimensioned having a diameter tocontact a portion of a wall of a vessel to thereby maintain the pressuresensor at a fixed location within the vessel.

24. The implantable medical device of paragraph 23, wherein the vesselis a pulmonary artery.

25. The implantable medical device of any of paragraphs 19-24, whereinthe housing further comprises an electronic circuit configured togenerate a signal indicative of the physiological parameter measured bythe sensing element.

26. The implantable medical device of any of paragraphs 19-25, whereineach of the fixation members includes first and second free ends withthe first and second free ends being oriented in opposing directionsrelative to one other.

27. The implantable medical device of any of paragraphs 19-26, whereinthe structure has a variable pitch such that the fixation assembly iscompressible in a delivery configuration while the physiological sensoris disposed within the delivery catheter and expandable into adeployment configuration that is different from the deliveryconfiguration responsive to withdrawal of the physiological sensor fromthe delivery catheter.

28. The implantable medical device of any of paragraphs 19-27, whereineach of the asymmetric fixation members is configured to contact thewalls of a blood vessel along a plurality of planes.

29. The implantable medical device of paragraph 28, wherein each planeof the plurality of planes is perpendicular to a surface of the capsuleto which the fixation members are affixed.

30. The implantable medical device of any of paragraphs 19-29, whereineach of the fixation members includes first and second free ends,wherein the first and second free ends are parallel with a plane of theplurality of planes.

31. The implantable medical device of any of paragraphs 19-29, whereineach of the fixation members includes first and second free ends,wherein the first and second free ends are parallel with a longitudinalaxis of the implantable medical device.

32. The implantable medical device of any of paragraphs 19-31, wherein asurface of the housing that contacts a wall of a blood vessel issintered.

33. The implantable medical device of any of paragraphs 19-32, whereinthe asymmetric fixation members have opposite arrangements as reflectedacross a central plane of the implantable medical device.

34. An implantable medical device, comprising:

a housing having first and second opposing ends;

a pressure sensing element on the housing;

an electronic circuit within the housing, the electronic circuit coupledto the pressure sensing element and configured to generate a signalindicative blood pressure; and

a fixation assembly including a first asymmetric fixation member coupledto the first opposing end of the housing and a second asymmetricfixation member coupled to the second opposing end of the housing,

wherein each of the asymmetric fixation members includes a structurewith a first loop and a second loop, the first loop more proximate tothe housing than the second loop,

wherein the first loop has a maximum pitch less than a maximum pitch ofthe second loop, and

wherein each of the fixation members includes first and second free endswith the first free ends of the fixation members being oriented inopposing directions relative to one other and the second free ends ofthe fixation members being oriented in opposing directions relative toone other.

What is claimed is:
 1. An implantable medical device configured toposition within a blood vessel, the implantable medical devicecomprising: a housing including a power source, a sensing element, andan electronic circuit configured to generate a signal indicative of aphysiological parameter measured by the sensing element; and a fixationassembly configured to be expandable in the blood vessel from a deliveryconfiguration to a deployment configuration, the fixation assemblyincluding a fixation member coupled to an end of the housing, whereinthe fixation member includes a first loop and a second loop, and whereinthe first loop and the second loop are configured to position thehousing adjacent a wall of the blood vessel when the fixation assemblyexpands to the deployment configuration.
 2. The implantable medicaldevice of claim 1, wherein the first loop and the second loop areconfigured to contact the wall of the blood vessel when the fixationassembly expands to the deployment configuration.
 3. The implantablemedical device of claim 1, wherein the housing defines a toplongitudinal wall and a bottom longitudinal wall opposite the toplongitudinal wall, wherein the sensing element is supported by the toplongitudinal wall, and wherein the fixation assembly is configured toposition the bottom longitudinal wall adjacent the wall of the bloodvessel when the fixation assembly expands to the deploymentconfiguration.
 4. The implantable medical device of claim 1, wherein:the fixation member is a first fixation member and the end of thehousing is a first end of the housing, the fixation assembly includes asecond fixation member coupled to a second end of the housing oppositethe first end, and the second fixation member is configured to positionthe housing adjacent the wall of the vessel when the first loop and thesecond loop position the housing adjacent the wall of the vessel.
 5. Theimplantable medical device of claim 4, wherein the first fixation memberand the second fixation member are configured to contact the wall of theblood vessel when the fixation assembly expands to the deploymentconfiguration.
 6. The implantable medical device of claim 1, wherein thefixation member is configured to occupy a plurality of planes when thefixation assembly expands to the deployment configuration, wherein theplurality of planes are configured resist a twisting of the housing in adirection perpendicular to one of the planes.
 7. The implantable medicaldevice of claim 1, wherein the fixation member includes a linear elementconfigured to form the first loop and the second loop.
 8. Theimplantable medical device of claim 1, wherein the fixation memberoverlaps at a first junction to form the first loop and overlaps at asecond junction to form the second loop.
 9. The implantable medicaldevice of claim 8, wherein a first pitch of the first loop increasesfrom the first junction in a direction away from the first junction, andwherein a second pitch of the second loop increases from the secondjunction in a direction away from the second junction.
 10. Theimplantable medical device of claim 1, wherein the fixation memberincludes a first free end and a second free end, and wherein thefixation member is configured to form the first loop and the second loopbetween the first free end and the second free end.
 11. The implantablemedical device of claim 1, wherein the implantable medical devicedefines a first channel and a second channel, and wherein the fixationmember is configured to define the first loop and the second loop when afirst segment of the fixation member is received in the first channeland a second segment of the fixation member is received in the secondchannel.
 12. The implantable medical device of claim 11, wherein thehousing includes a first longitudinal wall and a second longitudinalwall defining a longitudinal axis, and wherein at least one of a firstfree end of the first segment or a second free end of the second segmentare substantially perpendicular to the longitudinal axis when the firstsegment is received in the first channel and the second segment isreceived in the second channel.
 13. The implantable medical device ofclaim 1, wherein the first loop defines a first helical configurationand the second loop defines a second helical configuration.
 14. Theimplantable medical device of claim 9, wherein the vessel is a pulmonaryartery.
 15. The implantable medical device of claim 1, wherein theelectronic circuit is configured to generate a signal indicative of thephysiological parameter measured by the sensing element.
 16. Theimplantable medical device of claim 1, wherein the first loop and thesecond loop are configured to position a surface of the housing adjacentthe wall of the blood vessel, and wherein the surface of the housing issintered.
 17. The implantable medical device of claim 1, wherein thesensing element is a pressure membrane and measured physiologicalparameter is a blood pressure.
 18. An implantable medical deviceconfigured to position within a blood vessel, the implantable medicaldevice comprising: a housing including a power source, a sensingelement, and an electronic circuit configured to generate a signalindicative of a physiological parameter measured by the sensing element;and a fixation assembly configured to be expandable in the blood vesselfrom a delivery configuration to a deployment configuration, thefixation assembly comprising: a first fixation member coupled to a firstend of the housing, wherein the first fixation member includes a firstloop and a second loop; and a second fixation member coupled to a secondend of the housing opposite the first end, wherein the first loop, thesecond loop, and the second fixation member are configured to contact awall of the blood vessel when the fixation assembly expands to thedeployment configuration, such that the first loop, the second loop, andthe second fixation member position the housing adjacent the wall of theblood vessel when the fixation assembly expands to the deploymentconfiguration.
 19. The implantable medical device of claim 18, whereinthe first fixation member includes a linear element configured tooverlap at a first junction to form the first loop and overlap at asecond junction to form the second loop.
 20. The implantable medicaldevice of claim 18, wherein the implantable medical device defines afirst channel and a second channel, and wherein the first fixationmember is configured to define the first loop and the second loop when afirst segment of the first fixation member is received in the firstchannel and a second segment of the first fixation member is received inthe second channel.